The priority benefit of the Apr. 4, 2002 filing date of provisional application 60/369,606 is hereby claimed.
1. Field of the Invention
The present invention relates to digital encoder for video images. In particular, the invention relates to a digital encoder that emulates the non-linear characteristics of film.
2. Description of Related Art
One of the defining characteristics of film is that it has a photonic response characteristic that is “S”shaped, known as the “D-logE” curve, which is a log plot of the density of the exposed film vs. the exposure required to achieve that density. This curve is analogous to the responsivity characteristic of a silicon image sensor which plots the level of exposure to light required to achieve a given output voltage. See FIG. 3. The primary difference between film and video is that conventional video encoding has implemented a relatively sharp knee, or “hard” saturation which means that image detail in highly exposed areas is lost whereas the “soft” characteristics of film provide image detail in these highlight areas. Similarly, in low light areas, silicon sensors provide a linear response down to practically zero output voltage where again film provides a soft “toe” thereby preserving “shadow detail”.
FIG. 3 is a graph of a representative photonic response of film and a silicon based sensor. Cinematographers use this characteristic of the film (each film stock is different) to achieve certain desired artistic effects and those that have experimented with digital HDTV cameras have consistently noted this particular shortcoming of digital systems.